Update AIF explainer example with the latest knative API.

[Tomcli]

/kind bug

What steps did you take and what happened:
Since KServe is updated with a later version of knative. Knative service v1alpha1 is not longer supported, so we need to update the AIF example to use the latest knative API.

What did you expect to happen:

Anything else you would like to add:
[Miscellaneous information that will assist in solving the issue.]

Environment:

• Istio Version:
• Knative Version:
• KFServing Version:
• Kubeflow version:
• Kfdef:[k8s_istio/istio_dex/gcp_basic_auth/gcp_iap/aws/aws_cognito/ibm]
• Minikube version:
• Kubernetes version: (use kubectl version):
• OS (e.g. from /etc/os-release):

[yuzisun]

@Tomcli Let's move the doc issue to the website repo.